一个平行四边形的底是18cm，高是10cm，它的面积是 ，和它等底等高的三角形的面积是 ．
× = ×= × =1．
35× =0.5× =× =6÷ =1．
任何一个数（零除外）与它的倒数的乘积是1． ．
× =× =× =9× =1．
和 互为倒数．
× = ×
= × =1．
× =6÷ =1．
× =
× =
× =9× =1．
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